Automatic Exposure Control (AEC) apparatus are widely used in conventional diagnostic X-ray equipment to control X-ray exposure levels received by a patient. Using an AEC device can help to limit the amount of radiation that is received by sensing the radiation level at a suitable location in the exposure path and providing an output signal that indicates when sufficient radiation has been received. This output signal is then used to disable power to the X-ray emission components, thereby stopping the generation of ionizing radiation.
There are often practical and physical difficulties in positioning an automatic exposure control (AEC) device when imaging patients in an intensive care unit (ICU). Therefore, such AEC devices, while commercially available, are rarely used in many ICU environments. Instead, the technologist generally uses an estimate based on experience with different types of imaging and on the patient's overall build. Instead of using the feedback available from AEC devices, the radiographic technologist manually sets exposure technique parameters (kVp and mAs) on the portable x-ray machine.
It is known that there is a significant degree of variability in the manual selection of exposure techniques, as it is the technologist who visually assesses the thickness of the patient before making the selections. The combination of the patient thickness, the type of imaging receptor (e.g., computed radiography (CR), or digital radiography (DR)), and the choice of exposure techniques directly influences both the noise appearance and contrast in the captured image.
Once the image is captured, the technologist makes a visual assessment of the image quality, and may also refer to an exposure indicator (EI) to determine if the image was properly exposed. The exposure indicator is a figure of merit that is calculated for the captured image and that is related to the average signal level for the anatomical region of interest.
The technologist may decide to repeat the image if the EI is too low, or if the image appears noisy, i.e., if the image is deemed to be underexposed.
To reduce the number of images that may need to be repeated because of underexposure, and because there is some variability associated with the choice of exposure parameters, it is a typical practice to set the exposure parameters well-above the minimum level that is required to produce a diagnostic quality image. Consequently, patients that are imaged using portable x-ray machines may often receive a considerably higher radiation dose than that which is required for diagnosis. This can be a particular problem in intensive care units, where patients typically receive one or more chest x-rays per day, including pediatric and neonatal intensive care unit patients.
In mammography and other specialized radiography applications, one or more initial sampling exposures are sometimes obtained and assessed in order to help determine what technique settings are most appropriate for a particular patient. This can include settings such as kVp and mAs technique settings, for example. In such applications, at least one low-dosage initial exposure, sometimes termed a “scout view”, is first obtained, then examined by an operator or processed as a guideline to minimizing exposure or to targeting exposure for characteristic organ characteristics such as density or for particular regions of interest (ROI) of the patient's anatomy. This initial image is typically discarded once analysis is completed and the actual radiographic image of the patient is then obtained. While this method has some value, there are drawbacks. Operator assessment of the subject from a low-dosage image requires good judgment, complicates the image acquisition and processing workflow, and adds to the overall cost of radiographic imaging. The initial low-dose exposure adds to the overall amount of exposure required.
There is, then, a need for apparatus and methods that help to reduce the overall amount of exposure needed in the portable x-ray environment, without compromising the quality of images obtained.